I recently dove using the Japanese research submersible Shinkai 6500, and booted my laptop when we got to the ocean floor at 5750m depth. I do have photo documentation, and the dive itself is recorded in the JAMSTEC dive database.

This might be the deepest boot of gentoo linux ever. Linux is not used in the submersible, nor in DSRV Alvin (Woods Hole's equivalent), which anyway can't go that deep. It might be used in other deep sea equipment, but usually would be booted on the surface and descend to the depths while running. In my case I booted the laptop when we reached bottom.